Generally, a fuel injection control system of an internal combustion engine includes an electromagnetic driving fuel injection valve, and calculates a required injection quantity in correspondence to an operation state of the internal combustion engine, and drives the fuel injection valve to open with an injection pulse having a width corresponding to the required injection quantity so that fuel corresponding to the required injection quantity is injected.
For a fuel injection valve of an in-cylinder injection type internal combustion engine injecting high-pressure fuel into a cylinder, however, as illustrated in FIG. 5, linearity of a variation characteristic of an actual injection quantity relative to an injection pulse width tends to be reduced in a partial lift region (a region of a partial lift state, or a region of a short injection pulse width allowing a lift amount of a valve element not to reach a full lift position). In the partial lift region, the lift amount of the valve element (for example, a needle valve) tends to greatly vary, leading to a large variation in injection quantity. Such a large variation in injection quantity may degrade exhaust emission or drivability.
An existing technique on correction of a variation in injection quantity of the fuel injection valve includes, for example, a technique described in Patent Literature 1, in which a drive voltage UM of a solenoid is compared to a reference voltage UR being the drive voltage UM filtered by a low-pass filter, and an armature position of the solenoid is detected based on an intersection of the two voltages.
In the technique of Patent Literature 1, however, the unfiltered drive voltage UM (raw value) is compared to the filtered reference voltage UR: hence, the intersection of the two voltages may not be accurately detected due to influence of noise superimposed on the unfiltered drive voltage UM. In addition, the intersection of the drive voltage UM and the reference voltage UR may not exist depending on characteristics of the solenoid. It is therefore difficult to accurately detect the armature position of the solenoid. Hence, the technique of Patent Literature 1 cannot accurately correct the variation in the injection quantity of the fuel injection valve due to the variation in the lift amount in the partial lift region.